Why are you waiting for? If he doesn't know your issues, he's falling in love to the person you created, not yourself. I'm sure you'll be happier if he loves you the way you are. You should tell him after you read this. Good luck!
I struggle with mental illness, including depression, but my boyfriend doesn't know. The topic will come up eventually; I can't wear short-sleeve shirts because if I did, he'd notice my arms. How do I tell him and how long should I wait?
Better now than never
By being honest. If you want to base your "relationship" on truth and not lies and secrets .. Then trust him, and allow him to trust you.
If you feel that you are acting a certain way rather than just being yourself, then tell him. However, if you feel that being with him is helping you and that it may be too early in the relationship to tell him then continue as you are, you could try wearing some thick bracelets to hide your marks, Johnny Depp did that for years.
When you feel ready tell him, he'll either understand or he won't, but if he has spent time with you and realised that despite suffering mental health difficulties you are a great person then he'll be worth it. Statistically, in the UK 1 in 4 people will suffer mental health problems at some point in their lifetime, it isn't uncommon.